[FIX] gcc11,12

Author: eseiler

include/sharg/detail/poison_config.hpp

@@ -89,4 +89,29 @@ concept poison_config_valid = (sizeof(poison_config) == sizeof(config<validator_
 
 static_assert(poison_config_valid<>, "sharg::detail::poison_config must have the same members as sharg::config!");
 
+/*!\brief This is a workaround for compilers that do not implement CWG2518 (GCC 11, GCC 12).
+ * \ingroup parser
+ * \sa https://en.cppreference.com/w/cpp/language/if#Constexpr_if
+ * \sa https://cplusplus.github.io/CWG/issues/2518.html
+ * \details
+ * Before CWG2518, a (discarded) statement couldn't be false in every case, e.g.
+ * ```cpp
+ * template <typename option_type>
+ * void add_option(option_type)
+ * {
+ *     if constexpr (std::is_same_v<option_type, int>)
+ *     {
+ *         return;
+ *     }
+ *     else
+ *     {
+ *         static_assert(false, "Should never happen");                          // invalid before CWG2518
+ *         static_assert(dependent_false_v<option_type>, "Should never happen"); // valid
+ *     }
+ * }
+ * ```
+ */
+template <typename>
+inline constexpr bool dependent_false_v = false;
+
 } // namespace sharg::detail

include/sharg/parser.hpp

@@ -263,7 +263,7 @@ class parser
     template <typename option_type>
     void add_option(option_type &, detail::poison_config const &)
     {
-        static_assert(false, "Forgot sharg::config?");
+        static_assert(detail::dependent_false_v<option_type>, "Forgot sharg::config?");
     }
     //!\endcond
 
@@ -301,7 +301,7 @@ class parser
     template <typename option_type> // Template needed to prevent instantiation of this function if unused.
     void add_flag(option_type &, detail::poison_config const &)
     {
-        static_assert(false, "Forgot sharg::config?");
+        static_assert(detail::dependent_false_v<option_type>, "Forgot sharg::config?");
     }
     //!\endcond
 
@@ -352,7 +352,7 @@ class parser
     template <typename option_type>
     void add_positional_option(option_type &, detail::poison_config const &)
     {
-        static_assert(false, "Forgot sharg::config?");
+        static_assert(detail::dependent_false_v<option_type>, "Forgot sharg::config?");
     }
     //!\endcond